Lumpmod is a command line program, it can only be accessed through command prompt. That means you have to know certain commands to do what you want it to do. To run command prompt, go to the start menu, and click on run and type cmd and then click okay. Then you would have to find where you put lumpmod in command prompt, assuming that you know how to work in it.
If you want something more graphical, you can use either SLumpEd or XWE, which both put map headers into maps.